1. Field of the Invention
This invention relates to the field of aircraft or projectile guidance using divert propulsion means; the propulsion means using aerodynamic maneuvering or gas propulsion to adjust the flight path of a projectile in response to a command from a guidance system; the guidance system including an attitude determination and estimation means such as a GPS signal and configured to respond to signals from a first and second linear accelerometer for an estimation of the projectile's roll angle, and information from an additional third accelerometer for a pitch angle estimation.
2. Description of the Prior Art
The general problem to be solved is how to guide a gun-fired projectile onto a target with a known geographic location at lowest cost with reasonable reliability. This application addresses a portion of that problem by providing various divert propulsion means based on aerodynamic maneuvering and/or gas propulsion to adjust the flight path of a projectile in response to a command from a guidance system. This application also provides means for enabling the guidance system to determine appropriate commands to communicate to the propulsion system based on an estimate of the trajectory of the projectile from launch, the initial conditions of the projectile immediately after launch and the distance, direction and altitude to the idealized trajectory calculated for the projectile at the time of launch or firing.
Prior approaches to the problem to be solved used external aerodynamic surfaces, e.g., canards, controlled by a guidance system that relied upon only a GPS receiver and a turns counter. Unfortunately, however, external surfaces cause difficulties in launching the projectiles, e.g., from within a launch barrel, and the GPS signals may be vulnerable to jamming. Accordingly, where external aerodynamic surfaces, e.g., canards, are used, additional provisions must be made to minimize launch tube (barrel) interference and to accommodate large forces imposed on the canards due to projectile acceleration and aerodynamic drag. Furthermore, additional measures may need to be implemented to ensure reliable reception and processing of an uncorrupted GPS signal, e.g., protection of the GPS signal.
Additional approaches for controlling the aerodynamic surfaces have relied upon a GPS receiver, a turns counter, and a triax of gyros, which further increases the cost of the system. Yet, this approach remains vulnerable to corruption of the GPS signal because, as in the first approach described above, in the event the GPS signal is lost, no mechanism exists to account for any external forces that may act on the projectile throughout its flight.
Prior attempts to address the vulnerability to GPS signal jamming have focused on either preventing interference with the GPS signal or enabling operation with limited GPS data. Attempts to enable operation with limited GPS data have typically involved increasing the performance and/or functionality of the inertial instruments on board the projectile. For example, to address the inability of the above-mentioned second approach to account for the projectile's reaction to external forces if/when the GPS signal may be lost, additional accelerometers may be incorporated into the system to enable compensation for the projectile's reaction to external forces, i.e., the sensor package may comprise a complete IMU. Gyroscopic instruments for aircraft use are well known and available in a number of technologies such as iron rotor and tuned rotor gyros, ring laser gyros, multi-oscillator gyros, zero lock gyros (ZLG), fiber optic gyros, resonator gyros such as HRGs or hemispherical or tubular ceramic resonant gyros and the like.
Unfortunately, however, incorporation of additional gyroscopic instruments and/or accelerometers are considered much too costly because of the gyros already in the package. Moreover, use of such instruments imposes additional constraints on the operational envelopes of the projectiles. For example, gyroscopic instruments are typically subject to failure modes and uncertainties relating to launch accelerations in the range of 15,000-30,000 Gs. Further, use of these technologies usually requires that the vehicle carry at least one gyro in a gimbaled or strap-down arrangement with the attendant disadvantages of cost, weight and power dissipation.
Accordingly, a need exists for a system and process for guiding a projectile to a target while eliminating reliance on aerodynamic surfaces external to the projectile and while minimizing or eliminating the vulnerability of the system and process to interference with, e.g., jamming of, the GPS signal. Thus, it would be advantageous to have an improved, cost-effective system and process for providing projectile guidance in the presence of GPS signal jamming and/or with limited GPS data.